[R] Algorithm = "port" convergence codes

[Nakamura]

Hello all,

I am fitting a Gamma distribution to some data I have using nls(). The
function obviously runs into issues when using a 0 as a parameter value. I
understand the line alg = "port" can be used to set the lower bounds to
prevent this from happening. When I run the code I get the following:
Algorithm "port", convergence message: relative convergence (4). I have
attempted changing the starting parameter values and the paramter estimates
appear robust. 

The R help file mentions the code is "0" for converence. Should I be
concerned here that nls() is returning a local optimum here, or is this
quite normal?

Thanks in advance!
Rob

PS Please try to ignore the fact a distribution is being fitted to too few
data points!


##Code

Acancellata.caughtintraps = c(6,2,1,2,0,0,0,0,0,0,0)  
Acancellata.released = 1250 

distances = c(2,3,4,5,7.5,10,12.5,15,17.5,20,25)
totalseedscaught = sum(Acancellata.caughtintraps)
Pcatch = totalseedscaught/Acancellata.released

## Calculate cumulative seed numbers reaching each distance
cumsumcaught <- rev(cumsum(rev(Acancellata.caughtintraps)))

i = numeric(11)
i <- distances

## Gamma distribution
gamma.nls  <- nls(cumsumcaught ~ Pcatch*Acancellata.released*
                pgamma(distances,shape=a,scale=c,lower.tail=FALSE),
		## 'alg="port"' is required for lower bounds ## 
		    alg="port",
		    start = list(a=5,c=1),
		    lower=list(a=0.1,b=0.1))

print(gamma.nls)
print(summary(gamma.nls))

## Output

[1] "Gamma"
Nonlinear regression model
  model:  cumsumcaught ~ Pcatch * Acancellata.released * pgamma(distances,     
shape = a, scale = c, lower.tail = FALSE) 
   data:  parent.frame() 
     a      c 
7.5731 0.4399 
 residual sum-of-squares: 4.176

Algorithm "port", convergence message: relative convergence (4) 

Formula: cumsumcaught ~ Pcatch * Acancellata.released * pgamma(distances, 
    shape = a, scale = c, lower.tail = FALSE)

Parameters:
  Estimate Std. Error t value Pr(>|t|)  
a   7.5731     2.3786   3.184   0.0111 *
c   0.4399     0.1415   3.109   0.0125 *
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 

Residual standard error: 0.6812 on 9 degrees of freedom

Algorithm "port", convergence message: relative convergence (4) 



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